1. Field of the Invention
This invention relates to an electrophotographic image forming apparatus such as full color copying machines, color laser printers, and the like, and, more particularly, it relates to an image forming apparatus for forming images on a paper sheet by means of a plurality of photoconductive drums.
2. Description of the Related Art
Recently, digital full color copying machines have been developed and getting popularity on a demand for color images. As one of color copying machines, there is provided a Four-tiered photoconductive drum type full color copying machines which comprise four photoconductive drums. In this type of copying machine, four photoconductive drums are arranged in parallel with one another, and images in yellow, magenta, cyan and black are formed on the drums, respectively. A transfer material is carried by a conveyer belt and sequentially brought into contact with these drums, so that the toner images on the drums are transferred to the drums and overlapped with each other. Thus, a full color image can be obtained.
Upon forming an image other than a full color image by means of the copying machine, e.g., an image in black, toner images are not formed on the three drums for yellow, magenta, and cyan and only a black toner image is formed on the drum for black. Then, the black toner image is transferred to a transfer material, thereby obtaining a black image.
A problem with such a copying machine is that, even when a monochromatic image is formed, the drums and developing devices for the remaining three colors are also driven. In other words, the three drums and the associated developing devices that do not participate in forming a monochromatic image are unnecessarily and uneconomically operated. Thus, the drum surfaces are worn off by cleaning devices which are in contact with the drum surfaces, and the drums and the associated developing devices are fatigued as they contact with each other. Additionally, the developing sleeves of the developing devices may remarkably be worn and the toner may quickly be degraded as the sleeves are in contact with the toner.
In order to solve the above problems, Japanese Patent Application KOKAI Publication No. 63-8658 discloses an image forming apparatus in which only an image carrier and a developing device associated therewith used for image formation are driven and another image carriers and developing devices are stopped. However, this Publication does not concretely show drive means for driving the image carriers and the developing devices.
In Japanese Patent Application KOKAI Publication No. 4-76565 is disclosed an apparatus wherein selecting means for selectively driving a developing device is arranged on a drive shaft of an image carrier and the developing device is moved toward and away from the image carrier by the selecting means. However, in this apparatus, the image carrier is larger than that in the Four-tiered photoconductive drum type copying machine. Further, supporting members for the developing devices are rotated individually, thereby generating large vibrations and influencing formed images.
Japanese Patent Application KOKAI Publication No. 4-134463 discloses a mechanism for switching a transfer sheet clamp mechanism and a transfer roller. However, this mechanism is complex in structure and large in manufacturing cost.